The present invention is generally related to control and operation of power converter devices, and, more particularly, to circuits and techniques for parallel operation of power converters.
Switching-mode power converters are widely used in numerous applications to meet the needs of electronic systems. For example, in the telecom and networking industries, DC/DC converters convert a raw dc voltage (input), usually over a certain variation range, to a dc voltage (output) that meets a set of specifications.
DC/DC converters are commonly paralleled at their outputs either to provide higher output power to a load or to provide redundant operation in high reliability applications where the output must be maintained within specification in the event of a failure of a DC/DC converter. One of the main factors that contribute to effective operation of parallel DC/DC converters is the current sharing mechanism that is implemented in the parallel system. The purpose of this mechanism is to ensure equal distribution of current among the devices.
Various types of circuits for sharing a common load among a plurality of DC/DC converters are known. In a traditional current share scheme, each converter module has a current-share terminal, either at the output or at the input. When converters are in parallel operation, these current-share terminals are tied together. The signal at the current-share terminal tries to maintain an almost equal current in each module, which could be determined by a master-slave mechanism or an average type of mechanism. The master-slave technique may include, for example, a dedicated-master scheme where one module is selected as the master or an automatic-master where the system decides which converter will be chosen as the master depending on which converter has the largest output current.
Feedback loops exist in switching power converters to help the system maintain a constant output voltage. For parallel operation, inside each module, the actual current feedback signal is used to adjust its output voltage reference so that all modules will share the load current. However, if the output voltage reference and the current sharing terminal are located at different sides of the isolation boundary inside the converter, the current-sharing circuit becomes complex since the control signal has to be passed through the isolation boundary. This is especially problematic in standard converter modules, such as half-brick modules, quarter-brick modules, xe2x85x9th brick modules etc, where board space is limited.
Accordingly, it would be desirable to provide a simple and effective solution without sending a signal across the isolation boundary.
In addition, for protection and control needs, the output current or the main switch current of the converter needs to be reliably sensed. In power converters, a current sense transformer is usually used to sense the current information in the power switches or in transformer windings, for current mode control and for over-current protection. FIG. 1 shows a forward converter with a current sensing transformer T_sen. The main purpose of T_sen is to produce, from the primary current, a proportional secondary current that can easily be measured or used to control various circuits. The primary winding is connected in series with the source current to be measured, while the secondary winding is normally connected to a meter, relay, or a burden resistor to develop a low level voltage that is used for control purposes. Whenever a current sensing transformer is used, the proper reset of the transformer core under all operating conditions must be ensured, otherwise the saturation of the core could lead to a distorted current information and therefore the control loop and protection will not function properly. The resistance of the reset resistor R_reset is much higher than the resistance of the sensing resistor R_sen so that the small magnetizing current in the sensing transformer can generate enough voltage to reset the core during a fraction of the xe2x80x9cOFFxe2x80x9d period of the main power switch Q1.
This current sensing scheme assumes the current in Q1 is always positive. However, in reality this current could be negative depending on the magnetizing current in the main transformer T and the output inductor current in Lo. This issue becomes more problematic in a dc/dc converter using synchronous rectification where there is a negative current in Lo under light load or during dynamic process during the xe2x80x9cONxe2x80x9d period of the main power switch Q1. Due to the diode D_sen, the negative current reflected to the sensing transformer output will create a high voltage on the resistor R_reset, which in turn causes a high magnetizing current in the sense transformer T_sen. This magnetizing current causes false signal at Vsense, and can bringing the converter into malfunction. This high voltage could also quickly saturate the current sensing transformer core and cause the damage to the converter due to loss of sensed signal. Over-current protection is also important in any dc/dc converter. As the output current reaches a predetermined level, the converter should shut down or enter into a constant power mode to prevent damage to the converter and the loads that it powers.
Accordingly, a simple and reliable current sensing scheme, which also provides over-current protection is desired.
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a power distribution system including a plurality of power converter modules each having a current sharing signal terminal on an input side and power output terminals on an output side, the corresponding power output terminals of the several modules being connected together and adapted to power a common load; an interconnecting signal bus coupled across the current sharing signal terminals on the input side; a plurality of feedback circuits, each of which is associated with one of said modules, each feedback circuit including a comparator (output error amplifier) for comparing a feedback voltage on the output side with a reference voltage to provide an error signal to the input side; the error signal conditioned to provide a current command signal to said signal bus, wherein the signal bus provides a common current command signal to drive the power converter modules.
In a specific aspect thereof, the feedback circuits include isolation circuitry to electrically isolate the error signal from the input side in the form of an opto-isolator apparatus. The output error amplifier drives the input of the opto-isolator apparatus. Moreover, the error signal is conditioned by a first buffer (a first operational amplifier) to provide the current command signal to the signal bus. To operate as a master-slave scheme, a diode may be series coupled to the output of the first operational amplifier such that the highest current command signal of all power converter modules is provided to the signal bus.
In a further aspect thereof, a second buffer (a second operational amplifier) is provided to condition the common current command signal from the signal bus prior to driving the power converter module associated with the second buffer. The power converter module is driven by a pulse-width modulated (PWM) controller having the output of the second operational amplifier as an input thereto. The PWM controller may also include a ramp compensation signal input and a current sense input. In another aspect thereof, the second buffer is a compensator that compares the common current command signal from the signal bus with a sensed signal related to output current. The output of the compensator drives the power converter module associated therewith. In a further aspect thereof, the first operational amplifier compares the error signal with a second reference voltage to provide the current command signal provided to the signal bus, wherein the second reference voltage is generated from a bias voltage or a reference voltage from a pulse-width modulated (PWM) controller. Optionally, a time delay (e.g., an R-C circuit) is introduced to the second reference voltage.
The present invention also provides a current share circuit for power converters in parallel operation. The circuit includes an interconnecting signal bus coupled across current sharing signal terminals on an input side of said power converters; a plurality of feedback circuits, each of which is associated with one of said converters, each feedback circuit including a comparator for comparing a feedback voltage on an output side of the power converter with a reference voltage to provide an error signal to the input side; the error signal conditioned to provide a current command signal to said signal bus, wherein the signal bus provides a common current command signal to drive the power converters.
Moreover, a method for current sharing in parallel operated power converters is provided, the method including (a) interconnecting a signal bus across current sharing signal terminals on an input side of said power converters; (b) providing a plurality of feedback circuits, each of which is associated with one of said converters, each feedback circuit including (i) comparing a feedback voltage on an output side of the power converter with a reference voltage to provide an error signal to the input side; (ii) conditioning the error signal to provide a current command signal to said signal bus, and (iii) providing a common current command signal from the signal bus to drive the power converters.
In a further aspect thereof, a current sense circuit for a power converter is provided including a current sense transformer generating a current indicative of the current through the main switch of the power converter; and a transistor synchronized with the main power switch having a first port coupled to the current sense transformer for receiving a voltage that is indicative of the current through the main switch of the power converter and a second port for providing an output voltage across a sense resistor that is indicative of the current through the main switch.
The sensed current can be used for the over-current protection of a power converter. The over-current protection circuit includes a first diode to sample and hold the peak value of the current sense signal; and comparison circuitry capable of comparing said peak value with a reference voltage and developing an over-current protection signal in accordance therewith.